7M heavily missfiring

[jak]

Hey guys.

im trying to fix my 7mge cressida which is giving me some grief.

It is missing on more than one cylinder and i cant figure out what the hell is wrong with it.

Ive had it fixed but its come back. when the miss first came around, i just changed all the regular stuff, plugs and leads. That did nothing. Pulled the dizzy cap off and the carbon pin in the centre of the cap was no longer and appeared to have got some serious heat into it. When it was missing really badly fuel was pissing out the exhaust (has exhaust off at extractors atm) like 100ml every 2 mins. like there was a puddle of fuel on the ground. My initial thought was that it was an injector stuck open but they were all clicking nicely.

I changed the cap and the problem was much better. its still appeared to be running rich tho with alot of black smoke out the exhaust. i thought all the fuel that was pissing out may have destroyed the oxygen sensor, so i changed it and it ran fine. i came to the conclusion that because the spark from the coil was having to transmit through the spring that normally holds the pin in the dizzy cap instead of the pin itself, it was causing a weak spark accross all cylinders and harldy any fuel was getting burnt.

i have just done a w58 manual conversion and while i was waiting for my modified tailshaft, i was starting it everyday and it was running fine. I got my tailshaft back, put it in, went to take it for a test drive and after a few mins the miss came back, but slowly. like it was running fine, then started missing a bit then more and more and now it has the same symptoms as before with all the fuel pissing out.

I have changed:

plugs
leads
afm
unplugged the tps and it seems to be ok
another o2 sensor
the dizzy cap is still ok
ignitor
coil
condensor
checked timing
ecu
distributor itself
rotor button

it is missing on 2 or 3 cylinders and i am now stumped. The car was converted to gas, but is dual fuel and i am fixing it on the fuel. I am conviced its a spark issue as you change it over to gas and nothing happens, still misses and carries on. The gas system was installed by some dodgy bloke and was giving me trouble before this all started. It appears to have a piggy back ecu to run the gas and im beginning to think that if its all wired in with the ecu that it could be causing the problem.

I was thinking it could be a dodgy fuel pump relay but even it the fuel pump was stuck on or something it would just go back down the return and into the tank.

Any advice or previous experience is appreciated.

Cheers
Jak

[neal]

have you checked any of your wiring? Any ECU codes?

[woggin]

Highly unlikely but have you looked at chassis grounds? block grounding to the chassis, as i said i highly doubt it would do anything, got me stumped!

it seems the only thing that you havent changed is the loom, look for fraying and broken connections!!
also, now that i think of it, the mx83 project car i have going at the moment had a miss, it was caused by one of 3 things:

The clip that holds the dizzy plug in was broken, wobbled loose, caused a miss which would kill motor.
The coil wiring was dodgy, not making good contact.
Spark plugs were gapped to high, not enough voltage to jump the gap!

[LeeRoy]

Is the plug gap right?

Had much wet weather lately?

Busted FPR?

[Jin~Jah]

check there isn't a whole lot of liquid in the spark plug wells, and maybe do a compression test. I had a BHG that blew between cylinders once. It gave the symptoms of 2x cylinders misfiring.

normally, checking for BHG would be a bit excessive, but... it's a 7m

[2jzhilux]

Busted FPR?

This

or i would be checking that the fuel cut off solenoid is working.
When you switch it to gas is it still injecting fuel as well or is it cutting out like it should.

[andrew_mx83]

mate do a comp test it will quickly tell u if the problem is mechanical or electrical.

does it have oil leaks from the rocker covers (it should, i believe this is factory spec )? as this will mean bad contact from the leads to the plugs

if not that then there is always the BHG gremlin, does it use any water? (comp test to confirm)



failing that, since u mentioned LPG it could always be a burnt valve, again a comp test will tell u this.
almost straight after putting my mx83 on gas it burnt a valve, lost comp on 1 cyl and had a shocking miss. was getting all gee'd up to build a new head for it and one day it just fixed itself, i have no idea how. all i know is it went back upto 220psi across the board.

is the fuel system definately not active when running LPG? could just be flooding it with 2 types of fuel


also, what are you doing fucking around with the AFM when ur on gas? delete it altogther and run straight gas FTW, big improvent in power & economy to be had here.

[Mr_Flawless]

Do a comp test, does it blow white smoke?

[andrew_mx83]

lots of little spots of black in the exhaust = burnt valve btw

[brett_celicacoupe]

1. put a timing light on each spark plug lead. check for firing consistency.

2. remove spark plugs (also disconnect coilpack) , run fuel pump for 10 mins without engine running. then crank over. if one or 2 injectors are leaking you may see the fuel puff out of the spark plug hole. if everything seems equal you will need to look deeper.

this should be a good starting point to make a diagnosis